A Revolution in Plate Tectonics?

Chiloe: so now that I've read more of this thread, your questioning whether there is any evidence of magma that originates in the lower mantle (below 660 km)? correct?

Got perovskite?

I hadn't thought of the plume model as the dominant model. But then I'm not in the field.

Most of what I knew about this debate came from Anderson and Natland, "A brief history of the plume hypothesis and its competitors" (2005):

Good to see that Warren Hamilton is still alive and kicking

Evolution of the Archean Mohorovicic discontinuity from a synaccretionary 4.5 Ga protocrust

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s–1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~ 4.5 Ga. We know little about Hadean crustal geology (> 3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era.

The mantle sample and Archean (< 3.9 Ga) crustal geology integrate well. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~ 2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That reenrichment enabled modern-style plate tectonics after ~ 600 Ma, with a transition regime beginning ~ 850 Ma.

Chiloe: just read the Anderson & Natland paper. The high resolution 3D Vs tomography supports large-scale, mushroom-like thermal anomalies beneath spreading ridges & "hot spots" rather than narrow, sheet- or pipe-like features.

Still not sure how this model explains kimberlite pipes, diamonds, perovskite, etc. I guess kimberlite pipes are associated with continental interiors; whereas, this paper is dealing with the mantle beneath oceanic crust.

I'm only half a dozen papers into this field but I think it's a glaring omission to not mention a mechanism for the break-up of Pangea or Gondwanaland, in discussing magma sources for hot-spots.

It's fine to offer up some "observation" that mantle plumes might be shallow sourced, but using seismic tomography as the major evidence is pretty weak.

It is generally accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming.

Hawaii, we thought we knew you

The classic Plate Tectonics story for Hawaii now appears to be mistaken. The Pacific plate did not suddenly change directions, and there is no deep, stationary plume. Other explanations fit the evidence better.

The original idea
In 1963 Wilson suggested that the Hawaiian Islands formed as the Pacific plate moved over continuously upwelling hot mantle. In 1968 Christofferson conjectured that the Emperor-Hawaii elbow records a change in direction of the Pacific plate over a fixed "hotspot". In 1971 Morgan speculated that the heat source is a stationary "plume" rising vertically through the deep mantle, and this has been widely accepted.6

Challenging the original idea
"Simple tests falsify conjecture that the...Emperor-Hawaii system formed above a stationary plume. As this is the only testable purported plume, global speculation favoring fixed plumes falls with it."6

1) Pacific plate motion - no rapid change in direction
The concept requires "an enormous reorganization of Pacific Ocean plates 45 million years ago."6 This is not found anywhere "along the northeast, east, or south sides of the Pacific plate," where crust of that age is preserved.6 "The Pacific-Antarctic Ridge is the key link... tying the relative motion of the oceanic plates of the Pacific basin to the rest of the world."2 A survey of the Pitman Fracture Zone along this ridge was carried out in 1992. The authors concluded that "predictions of the track of the Hawaiian hot spot based on global reconstructions fail, once again, to predict a large bend around 43 million years ago."2 Next, "the Gilbert Ridge and Tokelau Seamounts are the only seamount trails in the Pacific Ocean with a sharp 60° bend, similar to the HEB." "The Louisville seamount trail is not useful... because it shows only a very broad curvature at its bend." Their study found that the Gilbert Ridge bend formed around 67 million years ago, while the Tokelau bend formed about 57 million years ago. They should have formed at the same time as the HEB, around 47 million years ago, "if they were formed by stationary hot spots, and assuming Pacific plate motion only." "Such asynchronous bends cannot be reconciled with the stationary hot spot paradigm."9 In addition, "improved mapping of marine magnetic anomalies in the Pacific has failed to define the directional change at 43 million years ago." "There was also a general lack of circ#m-Pacific tectonic events documented for this time. Recent age data suggest a slightly older age for the bend, about 47 million years ago, but this revised timing still does not correspond to an episode of profound plate motion change recorded within the Pacific basin or on its margins."15 "The textbook explanation for intraplate volcanism by fixed hot spots is either entirely wrong or insufficient to explain these phenomena."9 The most recent study of volcanoes of the central and southern Emperor chain claims that the Hawaii-Emperor Bend started at 50 million years ago.12 This is still far off of the 57 and 67 million years ago that are designated for the Gilbert Ridge and Tokelau Seamounts, and does not lessen the misfit between the predicted track of the Hawaiian hotspot, using magnetic anomalies at the Pitman Fracture Zone, and the actual island chain (below).

But you just said it wasn't moving?